1. Field of the Invention
The invention relates to electrical and optical devices that incorporate crystalline group III-nitrides.
2. Discussion of the Related Art
Crystalline group III-nitride semiconductors are used in both electrical devices and optical devices.
With respect to electrical devices, group III-nitrides have been used to make field-emitters. A field-emitter is a conductive structure with a sharp tip. The sharp tip produces a high electric field in response to being charged. The high electric field causes electron emission from the tip. For this reason, an array of field emitters can operate a phosphor image screen.
One prior art method has fabricated arrays of field-emitters from group III-nitrides. Group III-nitrides have chemical and mechanical stability due to the stability of the group III atom-nitrogen bond. Such stability is very desirable in devices that use an array of field-emitters.
The prior art method grows the field emitters from group III-nitrides. The growth method includes epitaxially growing a gallium nitride (GaN) layer on a sapphire substrate, forming a SiO2 mask on the GaN layer, and epitaxially growing pyramidal GaN field-emitters in circular windows of the mask. While the growth method produces field-emitters of uniform size, the field emitters do not have very sharp tips. Sharper tips are desirable to produce higher electron emission rates and lower turn-on voltages.
With respect to optical devices, group III-nitrides have high refractive indices. Materials with high refractive indices are desirable in the manufacture of photonic bandgap structures. For a fixed photonic bandgap, such materials enable making a photonic bandgap structure with larger feature dimensions than would be possible if the structure was made from a lower refractive index material.
One method for making a planar photonic bandgap structure involves dry etching a smooth layer of group III-nitride. Unfortunately, the chemical stability of group III-nitrides causes dry etchants to have a low selectivity for the group III-nitride over mask material. For that reason, a dry etch does not produce a deep surface relief in a layer of group III-nitride. Consequently, the dry-etch method only produces thin planar photonic bandgap structures from group III-nitrides.
Unfortunately, light does not efficiently edge couple to thin planar structures. For this reason, it is desirable to have a method capable of fabricating a photonic bandgap structure with a higher surface relief from a group III-nitride.